(1) Field of the Invention
This invention relates to an improved method for the production and use of ultrapure water, which is useful in fields such as fabrication of semiconductors, and also to an apparatus therefor.
(2) Description of the Prior Art
Ultrapure water, which is employed in fields such as fabrication of semiconductors, is now required to have still higher purity as the integration of semiconductors advances further. Ultrapure water is generally produced by processing raw water through a series of treatment steps which include a treatment step by a permeable membrane such as reverse osmosis membrane, a deionization step making use of an ion-exchange resin, a sterilization step by ultraviolet rays, an activated carbon treatment step, a treatment step by regenerable ion-exchange resins packed in the form of a mixed bed and a treatment step by a permeable membrane. The thus-produced ultrapure water is fed to a use site and its unused portion is recycled to a point either before or after the deionization step.
As various piping arranged between apparatus for practising the respective steps in the above production process and in a use system of ultrapure water, there are widely used pipes made of plastics such as hard vinyl chloride resin, polyvinylidene fluoride resin (hereinafter abbreviated as "PVDF") or tetrafluoroethylene-fluoroalkyl vinyl ether copolymer resins (hereinafter abbreviated as "PFA").
Use of these plastic pipes cannot however meet the above-mentioned demand for still further purification of ultrapure water, because organic substances are dissolved out and the growth of bacteria is promoted. In order to solve this problem, ultrapure water in a use system which follows the permeable membrane treatment step must be heated to 80.degree.-100.degree. C. or in some instances, even up to 120.degree. C. for its sterilization. The heat resistance and hot water resistance of conventional plastic pipes were too low to perform the above heat sterilization, so that a variety of problems arose. Included in such problems were deformation of hard PVC pipes, discoloration of PVDF pipes and deformation and embrittlement of PFA pipes.